1. Field of Invention
This invention relates to an electro-optical element driving circuit which drives an electro-optical element in an electro-optical device represented by a display device using an electro-optical substance such as liquid crystal, an electro-optical device using this driving circuit, and an electronic device. More specifically, this invention relates to an electro-optical element driving circuit which is integrally formed with an electro-optical element on the same substrate within an electro-optical device.
2. Description of Related Art
An electro-optical device which performs display processing or the like using liquid crystal or the like is generally known. In this type of electro-optical device, conventionally, it is common that an element portion including an electro-optical element which actually performs the display processing or the like and an electro-optical element driving circuit which drives this electro-optical element are individually fabricated and are mutually connected in a final process, thus forming a single electro-optical device. However, in an electro-optical device of this type of structure, after the element portion and the driving circuit which drives the element portion are individually fabricated, a separate process is needed to assemble them together, so there is a problem such that the process of fabricating the device becomes complicated, the cost becomes higher, and the entire electro-optical device becomes larger.
Therefore, integrally fabricating these components can be considered, but in this type of integral structure, if the number of electro-optical elements increases and the element portion is to have a micro-structure, a width of the wiring becomes narrow. Furthermore, arrangement of wires becomes complicated. Because of this, electric power consumption increases, and there is a problem such that the entire electro-optical device becomes larger.
Furthermore, it is desired that a driving circuit be digitally structured, but there are many cases that an electro-optical element is analog-driven. Therefore, if the driving circuit and the element portion are integrally formed, there is a problem such that noise generated from a circuit which performs digital processing leaks into a circuit which performs analog processing, and the electro-optical element cannot be accurately driven.
This invention reflects upon the above-mentioned problems, and an object of this invention is to provide an electro-optical element driving circuit, an electro-optical device provided with the electro-optical element driving circuit, and an electronic device which accomplish lower electric power consumption and decrease noise. Furthermore, these objects are to be accomplished even while simplifying a fabricating process by integrally forming an element portion including a plurality of electro-optical elements and a driving circuit which drives the respective electro-optical elements, decreasing the cost of the electro-optical device, and making the entire device smaller.
In order to solve the above-mentioned problems, this invention is an electro-optical element driving circuit which drives a plurality of electro-optical elements arranged in matrix. The invention has a plurality of digital processing means that correspond to respective bits in a digital driving signal for driving each of the electro-optical elements, and which generate a processing digital driving signal by performing predetermined digital processing to the digital driving signal. The invention also has a plurality of voltage increasing means that increase the voltage of the processing digital driving signals and generate increased voltage digital driving signals. The invention also has a plurality of digital/analog converting means that generate analog driving signals corresponding to a digital value included in the digital driving signal by digital/analog converting the increased voltage digital driving signal, and drive the respective electro-optical elements by applying the analog driving signals to the respective electro-optical elements.
In this invention, a plurality of digital processing means corresponding to respective bits of a digital driving signal perform predetermined digital processing to the digital driving signal and generate processed digital driving signals. Then, a plurality of voltage increasing means increase the voltage of the processing digital driving signals, and generate increased voltage digital driving signals. Furthermore, a plurality of digital/analog converting means digital/analog convert the increased voltage digital driving signals, generate analog driving signals corresponding to a digital value included in the digital driving signal, and drive the respective electro-optical elements by applying a voltage to each electro-optical element. Therefore, the processing voltage in the digital processing of the input digital driving signal can be lower than the voltage of the analog driving signal such that digital noise can be decreased, and the lower electric power consumption can be achieved for the entire electro-optical element driving circuit.
In this invention, each digital processing means and each voltage increasing means are disposed on a substrate such that a digital/analog converting means is sandwiched between an element portion, on which the plurality of electro-optical elements are disposed, on one side and the substrate on the other side. Each of the digital processing means, each of the voltage increasing means, and each of the digital/analog converting means are disposed in the area that is at a distance equal to an interval of the electro-optical elements from a signal line for supplying the analog driving signal to the electro-optical elements, in a direction perpendicular to the signal line. The digital/analog converting means are switch-capacitance type digital/analog converting means which comprise an analog switch and a capacitance circuit. Among the digital processing means, the voltage increasing means, and the capacitance circuit, it is preferable that the capacitance circuit is disposed at a position closest to the element portion.
With this structure, a circuit to perform digital processing and a circuit to perform analog processing are isolated, and all of the circuits are disposed within an area of a distance equal to an interval of the electro-optical element in a direction perpendicular to the signal line, so it is possible to decrease leakage of the noise from the circuit which performs digital processing into the circuit that performs analog processing.
In this invention, each capacitance circuit comprises a plurality of capacitance elements having capacitances which are mutually different in magnitude, corresponding to the respective digital values. The plurality of capacitance elements are preferably disposed at locations such that the capacitance element having the largest capacitance among all the capacitance elements is closest to the element portion, and as distance from the element portion increases, the capacitance becomes smaller.
With this structure, the capacitance element that occupies the largest area is disposed in an area in which there are the least number of wires, so each element can be disposed by effectively using the area on the substrate without waste.
In this invention, if n is a natural number, among the plurality of capacitance elements, a capacitance element corresponding to an nth bit (n is a natural number) of the digital value is preferably structured by 2nxe2x88x921 unit cells.
With this structure, irregularity of the process when a capacitance circuit with a large region is fabricated is considered, and it is possible to prevent accuracy deterioration of the digital/analog converting means corresponding to each bit of the digital driving signal.
In this invention, each analog switch is preferably structured by a thin film transistor. It is preferable that the channel width of the analog switch corresponding to nth bit of the digital value is 2nxe2x88x921 times a predetermined unit channel width.
With this structure, the ratio between the capacitance of the capacitance element for each bit and the channel width of the analog switch becomes constant, and thus a so-called punch-through voltage is constant and accuracy of the analog driving signal can be improved.
In this invention, it is preferable that the capacitance of each capacitance element is smaller than the capacitance it originally should have been with respect to the digital value by the amount of the capacitance between wires received by the capacitance element from wiring on another substrate.
With this structure, because the capacitance of each capacitance element is smaller than the capacitance it originally should have been with respect to the digital value by the amount of the capacitance between wires received by the capacitance element from wiring on another substrate, it is possible to prevent accuracy of the analog driving signal from deteriorating as the apparent capacitance of the respective capacitance elements increases due to the effect of the capacitance between wires.
In this invention, it is preferable that, on the substrate, a plurality of elements which respectively form each digital processing means, each voltage increasing means, each analog switch and each capacitance circuit are arrayed and disposed for each bit in a direction in which the signal lines are disposed, and further that wires among the plurality of wires connected between the respective elements that are parallel to the signal lines are mutually separated on the substrate, as seen from a plan view.
With this structure, because a so-called cross-over surface area of wires is minimized, there is no parasitic capacitance between wires. Therefore, lower electric power consumption can be achieved, and yield can be improved by preventing shorts during the fabricating process.
In this invention, it is preferable that, among a plurality of the wires parallel to the signal line, wires which are formed between different layers in multi-layer wiring are alternately arrayed and formed in a direction perpendicular to the signal lines. That is, if each wiring goes through a different interlayer insulating film, it is preferable that parallel wires are formed so as to not overlap each other, as seen from a plan view.
With this structure, wires which extend in parallel sandwich an interlayer insulating film and do not overlap each other, such that even if each element is closely disposed within a specified region, it is possible to prevent parasitic capacitance between wires. Thus, even lower electric power consumption can be achieved.
If an electro-optical device provided with an element portion where a plurality of electro-optical elements are disposed in a matrix is structured by using an electro-optical element driving circuit that incorporates this invention, the electro-optical device can be made smaller, accuracy can be improved, and lower electric power consumption can be achieved.
In this invention, by using a liquid crystal display element as the electro-optical element, a liquid crystal device can be smaller, accuracy can be improved, and lower electric power consumption can be achieved.
By installing this type of electro-optical device in an electronic device, the electronic device can be made to be smaller, accuracy can be improved, and lower electric power consumption can be achieved.